Age, period, and cohort effect

A model with age, period, and cohort can only be identified if:

1) Two or more of the remaining age, period, or cohort coefficients to be equal;
2) Use a proxy variable approach that assumes the cohort (e.g. cohort size) or period effects are proportional to certain measured variables;
3) Transform at least one of the age, period, or cohort variables so that its relationship to other is nonlinear.

A piecewise linear hazard rate model can usually be identified because of (3).